1. Field of the Invention
The present invention relates to a heat transfer sheet used in an image forming process in which an image with a high resolution is formed by using a thermal head or a laser light.
2. Description of the Related Art
As an image recording process by heat-transfer, conventionally, a process in which a heat transfer sheet is directly pressed and heated by a thermal head is in widespread use. This process has some superior characteristics in that it is able to be performed at a low noise level using a simple device structure, maintenance is not necessary, and the process is a dry processing. Recently, the densification of the thermal head itself has been improved. In the above process, it is possible to obtain images whose resolution has been increased to a high level.
As an image recording process by heat-transfer which is able to record images with higher resolutions, a laser heat transfer recording process has been known in which a laser light is irradiated onto the heat transfer sheet, the laser light is converted into heat in the heat transfer sheet and thus recording is carried out by the heat. In the above described process, because laser light serving as an energy supply source can be condensed to a few microns, it is possible to improve resolutions significantly compared with the process using the thermal head.
As the heat transfer sheet used in a laser heat transfer recording process, a heat-fusing transfer sheet (described in Japanese Patent Application Laid-Open (JP-A) No. 5-58,045 and the like) is known, which comprises, on a substrate, a light-to-heat conversion layer which absorbs the laser light and generates heat, and an image forming layer in which pigment is dispersed in a component such as a heat-fusing wax, a binder or the like in that order. When the above-described heat-fusing transfer sheet is used, the image forming layer corresponding to a laser light illuminating region of the light-to-heat conversion layer is fused by heat generated in the laser light illuminating region, and is transferred to an image receiving sheet superimposed on the heat-fusing transfer sheet. As a result, a transferred images formed on the image receiving sheet.
JP-A No. 6-219,052 disclosed an image forming process in which a heat transfer sheet is used for forming highly detailed image on the image receiving sheet which is superimposed upon the heat transfer sheet In the heat transfer sheet, a light-to-heat conversion layer containing a heat-to-light conversion material, a very thin (0.03-0.31 xcexcm) heat peeling layer and an image forming layer containing a color material are disposed in that order on a substrate, and the bonding strength between the image forming layer and the light-to-heat conversion layer bound together due to the heat peeling layer therebetween is weakened by the irradiation of laser light. In this image forming process, so called xe2x80x9cabrasionxe2x80x9d is utilized. Specifically, in a region illuminated with the laser light, the heat peeling layer is partially decomposed and vaporized. As a result, the adhesion between the image forming layer and the light-to-heat conversion layer is weakened so that the image forming layer in the illuminated region is transferred to the image receiving sheet superposed on the heat transfer sheet and the transferred image is formed on the image receiving sheet.
The image forming process utilizing laser light has advantages in that it is able to use printing paper provided with an image receiving layer (an adhesive layer) serving as the image receiving sheet, a multi-color image can be easily formed by transferring sequentially different color images onto the image receiving sheet and the like. In particular, the image forming process utilizing abrasion has an advantage in that highly detailed images can be easily formed.
In these image forming methods using heat transfer, because the adhesion between the heat transfer sheet and the image receiving sheet when the image is transferred affects considerably the resolution of the image, the key to obtaining an image with high resolution is enhancing the adhesion between the heat transfer sheet and the image receiving sheet
As a method for enhancing the adhesion between the heat transfer sheet and the image receiving sheet, there may be proposed a method in which the heat transfer sheet and the image receiving sheet are superimposed with each other, and are wound onto a cylindrical drum having a plurality of through holes. The pressure inside the cylindrical drum is then reduced such that the adhesion between the heat transfer sheet and the image receiving sheet is enhanced. However, in this process, air is accumulated between the heat transfer sheet and the image receiving sheet and the sheets becomes wavy. As a result, complete adhesion of the sheets may be rather inhibited. Therefore, in order to solve the problems such as air accumulation and the like, particulate material, having a particle size of several xcexcm to several tens of xcexcm, which is referred to as a matte agent, is added to surface layers of the heat transfer sheet and/or the image receiving sheet which face each other so that the surface of the heat transfer sheet and/or the image receiving sheet is made uneven. By this method, problems such as air accumulation are solved and vacuum adhesion uniformity between the sheets can be ensured.
However, when the matte agent is added to the surface layer of the heat transfer sheet, problems such as image voids may be generated and at the same time the hue of formed images may be affected so that images having good hues cannot be formed. On the other hand, when the matte agent is added to the surface layer of the image receiving sheet, there may be cases in which image voids are generated.
Therefore, an object of the present invention is to provide a heat transfer sheet which can ensure vacuum adhesion uniformity between the heat transfer sheet and an image receiving sheet at the time of heat-transfer and form images with high quality without using a matte agent which may affect the image quality in an ink layer.
In order to accomplish the above-described object, the present invention has the characteristics given below.
In accordance with a first aspect of the present invention, there is provided a heat transfer sheet which comprises a substrate, and an ink layer which is provided on the substrate and contains a pigment and an amorphous organic polymer, wherein the ink layer is structured such that, no particulate material having the function of forming unevenness of the surface of the ink layer is used in the ink layer, and the 10-point mean surface roughness (Rz) of the surface of the ink layer is 0.5xcx9c5.0 xcexcm.
In accordance with a second aspect of the present invention, an intermediate layer, containing a particulate material having the function of forming unevenness on the surface of the ink layer, is provided between the substrate and the ink layer.
In accordance with a third aspect of the present invention, the thickness of the intermediate layer portions, where no particulate material which has the function of forming unevenness on the surface of the ink layer is present, is smaller than the number average particle diameter of the particulate material.
In accordance with a fourth aspect of the present invention, the number average particle diameter of the particulate material is 0.5 xcexcm or more, and the ink layer contains a wax.
In accordance with a fifth aspect of the present invention, the weight ratio (W:P) of the content of the wax (W) to the content of the amorphous organic polymer (P) in the ink layer is 5:100xcx9c100:100.
In accordance with a sixth aspect of the present invention, the number average particle diameter of the particulate material is 0.6xcx9c4.0 xcexcm.
In accordance with a seventh aspect of the present invention, the content of the particulate material in the intermediate layer is 5xcx9c100 mg/m2.
In accordance with an eighth aspect of the present invention, the particulate material is organic particulates.
In accordance with a ninth aspect of the present invention, the entire heated ink layer is transferred during heat-transfer.
In accordance with a tenth aspect of the present invention, the intermediate layer contains a material having a light-to-heat conversion function.
In accordance with an eleventh aspect of the present invention, a cushion layer is disposed between the substrate and the intermediate layer and the thickness of the cushion layer is 1xcx9c30 xcexcm.
The heat transfer sheet of the present invention comprises at least an ink layer provided on a substrate wherein the ink layer does not contain a particulate substance having the function of forming unevenness on the surface of the ink layer. In addition, the 10-point mean surface roughness of the surface of the ink layer is between 0.5xcx9c5 xcexcm. Moreover, an intermediate layer, a cushion layer, or a heat-sensitive peeling layer may be provided where necessary either singly or in combination between the substrate and the ink layer in the heat transfer sheet of the present invention. A light reflux prevention layer may also be provided on the surface of the substrate opposite to that on which the ink layer is provided. Further, an image receiving sheet may also be superposed where necessary on the ink layer of the heat transfer sheet of the present invention.
A preferred embodiment of a heat transfer sheet of the present invention will be described in detail hereinafter with the explanation devided into a plurality of sections describing the substrate, ink layer, other layers which are provided as occasion demands, and the image receiving sheet. Also, an image forming method using the heat transfer sheet of the present invention will be described.
The material for the substrate is not particularly limited. Therefore, various materials can be used in accordance with purposes. Preferred examples of the material for the substrate include synthetic resins such as polyethylene terephthalate, polyethlene-2,6-naphthalate, polycarbonate, polyethylene, polyvinyl chloride, polyvinylidene chloride, polystyrene, and styrene/acrylonitrile copolymers and the like. Among these materials, biaxial oriented polyethylene terephthalate is preferable from its mechanical strength and dimensional stability with respect to heat When the heat transfer sheet of the present invention is used in a laser heat transfer recording method, the substrate of the heat transfer sheet is preferably made from a transparent, laser light-transmitting synthetic resin.
In order to improve the adhesion between the substrate of the heat transfer sheet and the layer provided on the substrate (i.e., an ink layer, an intermediate layer or a cushion layer), it is preferable to carry out a surface activation treatment of the substrate and/or to apply one layer or two or more layers of a primer to the substrate. Examples of the surface activation treatment include a glow discharge treatment, a corona discharge treatment, and the like. Materials for the primer preferably have good adhesion to the surfaces of both the substrate and the light-to-heat conversion layer, low thermal conducfivity and excellent heat resistance. Examples of such materials for the primer include styrene, a styrene/butadiene copolymer, gelatin and the like. The total thickness of the primer layer is generally 0.01 xcexcmxcx9c2 xcexcm.
In the heat transfer sheet of the present invention, an ink layer may be provided directly on the substrate. Alternatively, the ink layer may be provided with at least one of the intermediate layer, the cushion layer and the heat-sensitive peeling layer being interposed between the substrate and the ink layer when necessary. The ink layer contains a pigment, an amorphous organic polymer and optionally other components.
(Pigments)
Pigments can divided roughly into organic pigments and inorganic pigments. Organic pigments provide highly transparent films, while inorganic pigments are generally excellent in concealability. When the heat transfer sheet of the present invention is used for printing color correction, organic pigments are suitably used whose hues are identical or close to the yellow, magenta, cyan, and black which are generally used in printing ink. In addition to these pigments, metal powders, fluorescent pigments, and the like may also be used. Examples of the pigments suited for use in the heat transfer sheet include azo-based pigments, phthalocyanine-based pigments, anthraquinone-based pigments, dioxazine-based pigments, quinacridone-based pigments, isoindolinone-based pigments, and nitro-based pigments. Typical pigments according to hue are shown below.
1) Yellow Pigments
Hansa Yellow G, Hansa Yellow 5G, Hansa Yellow 10G, Hansa Yellow A, Pigment Yellow L, Permanent Yellow NCG, Permanent Yellow FGL, Permanent Yellow HR;
2) Red Pigments
Permanent Red 4R, Permanent Red F2R, Permanent Red FRL, Lake Red C, Lake Red D, Pigment Scarlet 3B, Bordeaux 5B, Alizarin Lake, Rhodamine Lake B;
3) Blue Pigments
Phthalocyanine Blue, Victoria Blue Lake, Fast Sky Blue;
4) Black Pigments
Carbon black.
In the present invention, the ink layer includes preferably 30xcx9c70% by weight and more preferably 40xcx9c60% by weight of pigments.
(Amorphous Organic Polymer)
Examples of the amorphous organic polymer included in the ink layer of the heat transfer sheet of the present invention and preferably having a softening point from 40 to 150xc2x0 C., include, for example, butyral resins; polyamide resins; polyethyleneimine resins; sulfonamide resins; polyesterpolyol resins; petroleum resins; homopolymers or copolymers of styrene, derivatives or substituents thereof, such as styrene, vinyltoluene, xcex1-methylstyrene, 2-methylstyrene, chlorostyrene, vinyl benzoate, soda vinylbenzenesulfonate, aminostyrene, and the like; homopolymers or copolymers of methacrylic esters such as methyl methacrylate, ethyl methacrylate, butyl methacrylate, hydroxyethyl methacrylate and methacrylic acid, acrylic esters such as methyl acrylate, ethyl acrylate, butyl acrylate, and xcex1-ethylhexyl acrylate and the like and acrylic acid, dienes such as butadiene and isoprene and the like, acrylonitrile, vinyl ethers, maleic acid, maleic esters, maleic anhydride, cinnamic acid, vinyl monomers like vinyl chloride, vinyl acetate and the like. These resins may be used as mixtures of two or more resins.
In the present invention, the ink layer contains preferably 70%xcx9c30% by weight and more preferably 60xcx9c40% by weight of amorphous organic polymers.
(Wax)
A wax is preferably contained in the ink layer of the heat transfer material of the present invention.
Examples of the wax include a solid or semi-solid wax having a melting point of 40xcx9c160xc2x0 C., for example, plant waxes such as Camauba wax, Japan wax and the like; animal waxes such as bees wax, whale wax and the like; oil waxes such as paraffin wax, polyethylene wax and the like; mineral waxes such as montan wax and the like; higher fatty acids such as behenic acid, stearic add and the like; higher alcohols such as palmityl alcohol, stearyl alcohol and the like; higher fatty acid esters such as cetyl palmitate and the like; higher fatty acid amides such as amide palmitate and the like; higher amines such as stearyl amine and the like.
Among these waxes, higher fatty adds and higher fatty add amides are preferable.
The weight ratio of the content of the wax (W) to the content of the amorphous organic polymer (P) in the ink layer (i.e., (W:P)) is preferably 5:100xcx9c100:100 and more preferably 10:100xcx9c70:100.
When the content of the wax (W) is too small, the transfer ability during heat-transfer deteriorates due to the particulate material contained in the intermediate layer so that sufficient image density cannot be obtained. When the content of the wax is too large, the adhesion resistance is decreased, and when the heat transfer materials are stored in a stacked manner, the ink layer may be transferred so that the image defects may be caused in the formed image.
(Other Components)
With the heat transfer sheet of the present invention, when the multi-color image is formed by superposing repeatedly multiple image layers (ink layers in which the images have been produced) on the same image receiving sheet, the ink layer preferably contains a plasticizer in order to enhance the adhesion between images. Examples of the plasticizer include phthalic esters such as dibutyl phtalate, di-n-octyl phthalate, di-(2-ethylhexyl) phthalate, dinonyl phthalate, dilauryl pthalate, butyl lauryl pthalate, butyl benzyl phthalate and the like; aliphatic dibasic acid esters such as di-(2-ethylhexyl) adipate, di-(2-ethylhexyl) sebacate, and the like; phosphoric triesters such as tricresyl phosphate, tri-(2-ethylehexyl) phosphate and the like, polyol polyesters such as polyethylene glycol ester and the like; and epoxy compounds such as epoxy fatty acid ester and the like. In addition to the above conventional plasticiziers, acrylic esters such as polyethylene glycol dimethacrylate, 1,2,4butanetriol trimethacrylate, trimethylolethane triacrylate, tenpaerythrite triacrylate, pentanerythrite tetraacrylate, dipentaerythrite-polyacrylate may be suitably used in accordance with the type of binder employed. These plasticizers may be used as mixtures of two or more plasticizers.
Generally, the plasticizer is used in the ink layer such that the weight ratio of the total amount of the pigment and the amorphous organic polymer to the amount of plasticizer is 100:1xcx9c100:3, and preferably 100:1.5xcx9c100:2. In addition to the above-mentioned components, a surfactant, a thickener and the like may be added to the ink layer as occasion demands. The thickness (dry layer thickness) of the ink layer is preferably 0.2xcx9c1.5 xcexcm, and more preferably 0.3xcx9c1.0 xcexcm.
In the heat transfer sheet of the present invention, it is necessary to ensure vacuum adhesion uniformity of the ink layer with the image receiving sheet by making the surface of the ink layer uneven to a certain degree. The degree of unevenness of the surface of the ink layer is represented by a 10-point mean surface roughness (Rz) and must be 0.5 xcexcmxcx9c5.0 xcexcm and preferably 0.5 xcexcmxcx9c3.0 xcexcm, and more preferably 1.0 xcexcmxcx9c2.5 xcexcm. When the 10-point mean surface roughness of the ink layer is below 0.5 xcexcm, the vacuum adhesion uniformity thereof with the image receiving sheet is insufficient, which is not preferable. On the other hand, when the 10-point mean surface roughness is more than 5.0 xcexcm, the tight adhesion to the image receiving sheet is lost and may lead to transfer unevenness, which is not preferable.
The 10-point mean surface roughness (Rz) is defined in JIS B0601 and is easily measured by a stylus type surface roughness tester, for example, surfcom 570A 3DF (manufactured by Tokyo Seimitsu Co. Ltd.).
The method for turning the surface of the ink layer into the state prescribed in the present invention is not especially limited. For example, a method in which the surface of the ink layer is embossed and a method in which an intermediate layer containing particulate material having the function of making the surface of the ink layer uneven, i.e., a matte agent, is interposed between the substrate and the ink layer may be used. Among these methods, the method in which an intermediate layer, which contains the particulate material having the function of making the surface of the ink layer uneven, is interposed between the substrate and the ink layer, is preferable. The specific structure of the method of providing the intermediate layer will be described hereinafter.
In the heat transfer sheet of the present invention, the intermediate layer can be provided between the substrate and the ink layer. The intermediate layer can be made so as to have one of or both of the functions described below: 1) the function in which the particulate material (matte agent) is contained in the intermediate layer as described above and the surface of the ink layer formed on the intermediate layer is made uneven as determined in the present invention (the unevenness-forming function); and 2) the function in which the light-to-heat conversion material is contained in the intermediate layer so that the energy from laser light is converted into heat, and therefore, the heat transfer of the ink layer becomes possible (the light-to-heat conversion function). A description will be given hereinafter of materials necessary for the above functions and of 3) a method of forming the intermediate layer.
1) Material Necessary for Unevenness-Forming Function
(Particulate Material (Matte agent))
As particulate material, inorganic particles or organic particles can be used. Organic particles are particularly preferable because they do not precipitate in the solvent due to their relatively small specific gravity and have good coating liquid stability. Specific inorganic particles include metallic salts of silica, titanium oxide, aluminum oxide, zinc oxide, magnesium oxide, barium sulfate, magnesium sulfate, aluminum hydroxide, magnesium hydroxide, boron nitride, and the like, and kaolin, clay, talc, zinc white, white lead, quartz, diatomaceous earth, perlite, bentonite, mica, synthetic mica and the like.
Specific examples of organic particles include resin particulates such as fluorocarbon resin particulates, PMMA particulates, polystyrene (PS) resin particulates, polyethylene (PE) resin particulates, guanamine resin particulates, benzoguanamine resin particulates, acrylic resin particulates, styrene-acryl copolymer resin particulates, silicon resin particulates, melamine resin particulates, epoxy resin particulates and the like. As the particulate material, other inorganic pigments and organic pigments can also be used.
Although the shape of the particulate material may be any shape so long as the surface of the ink layer can be made uneven, a spherical shape is especially preferable due to its stable performance.
The particulate material is desirably monodispersible in order to enhance the uniformity of the unevenness of the surface of the ink layer, and the particle diameter must be large enough to provide the desired unevenness on the surface of the ink layer. Preferable values for the particle diameter may be different depending on the thickness of the intermediate layer and/or the ink layer and the amount added of the particulate material.
Specifically, the particle diameter of the particulate material is, as the number average particle diameter, preferably 0.5 xcexcm or more and more preferably 0.6xcx9c4.0 xcexcm and most preferably 0.8xcx9c2.0 xcexcm.
When the number average particle diameter is less than 0.5 xcexcm, the air accumulation is caused when the heat transfer sheet is adhered to the image receiving sheet by vacuum pressure so that transfer unevenness may be caused.
Specific examples of the particulate material include, for example, the spherical silica particulates (1.2 xcexcm), the spherical silica particulates (2.0 xcexcm), the freeshaped silica particulates (1.8 xcexcm), the freeshaped aluminum oxide particulates (1.5 xcexcm), the spherical crosslinking PMMA particulates (1.8 xcexcm), the spherical cross inking PMMA-PS particulates (1.5 xcexcm), MX-150, 180,300 (PMMA particulates manufactured by Soken Chemical and Engineering Co., Ltd.), EPOSTAR S12 (melamine particulates manufactured by Nippon Shokubai Co., Ltd.), EPOSTAR MS (benzoguananiine particulates manufactured by Nippon Shokubai Co., Ltd.), EPOSTAR M30 (melamine/benzoguanarine particulates manufactured by Nippon Shokubai Co., Ltd.), SEAHOSTAR KE-P70, KE-P100, KE-P150 (melamine particulates manufactured by Nippon Shokubai Co., Ltd.) and the like.
If the thickness of the intermediate layer is made thinner, the particulate material tends to protrude from the surface of the intermediate layer. Especially, when the number average particle diameter of the particulate is larger than thickness of the portions of the intermediate layer where no particulate material is present, most of the particulate material contained in the intermediate layer protrudes from the surface of the intermediate layer, which is preferable for forming unevenness efficiently on the surface of the ink layer. The number average particle diameter of the particulate material is larger than the thickness of the portions of the intermediate layer where no particulate material is present by preferably by 0.05 xcexcmxcx9c10 xcexcm and more preferably 0.1 xcexcmxcx9c5 xcexcm. Of course, even if the number average particle diameter of the particulate material is less than the thickness of the portions of the intermediate layer where no particulate material is present, if the amount added of the particulate material is sufficiently large, the desirable unevenness on the surface of the ink layer can be formed.
The content of the particulate material varies depending on the number average particle diameter of the particulate material, the thickness of the intermediate layer and the like, and is preferably 5xcx9c100 mg/m2 and more preferably 5xcx9c50 mg/m2.
When the content is 5 mg/m2, there may be a case in which unevenness is not sufficiently formed on the surface of the ink layer. When the content is over 100 mg/m2, the transfer ability of the image is decreased so that the image quality is deteriorated.
It is desirable to make the thickness of the intermediate layer thinner than the number average particle diameter of the particulate material as described above, and specifically, the thickness of the intermediate layer is preferably 0.1xcx9c1 xcexcm and more preferably 0.15xcx9c0.8 xcexcm.
(Binder)
The intermediate layer having the unevenness-forming-function is formed by dispersing the above-described particulate material and a binder in an appropriate solvent to prepare a coating solution, applying the coating solution to the substrate or a cushion layer which will be described later, and then drying the coating solution. Examples of binders include polymers or copolymers of acrylic monomers such as acrylic acid, cellulose polymers such as cellulose acetate, polystyrene, vinyl polymers such as vinyl chloride/vinyl acetate copolymers, polyvinyl butyral, polyvinyl alcohol and the like, condensed polymers such as polyester, polyamide, polyimide and the like, rubber-based thermoplastic polymers such as butadiene/styrene copolymers, polyurethane, epoxy resin, urea/melamine resin and the like. Among these materials, polymers such as polyvinyl alcohol, polyvinyl butyral, polyester, polyimide and the like are preferably used. Especially preferable binders include the polyimide resin disclosed in Japanese Patent Application No. 10-14092.
2) Material Necessary for Heat-to-Light Conversion Function
(Light-to-Heat Conversion Material)
The light-to-heat conversion material usable in the heat transfer sheet of the present invention is not especially limited and any light-to-heat conversion materials which are known conventionally may be used. Conventionally known light-to-heat conversion materials are generally dyes which are able to absorb laser light (i.e., pigments and the like). Examples of such dyes (pigments) include black pigments such as carbon black, pigment of macrocyclic compounds having absorption from the visible range to the near-infrared range such as phthalocyanine, naphthalocyanine, organic dyes used as laser absorbing materials in high density laser recording such as optical discs (cyanine dye, anthraquinone-based dye, azulene-based dye, and phithalocyarune-based dye other than indolenine dye of the present invention), and organometallic compound dyes such as a dithiol/nickel complex. It is desirable that the light-to-heat conversion material has a high light-to-heat conversion capability. Especially preferable light-to-heat conversion materials include the indolenine-based compound disclosed in Japanese Patent Application No. 10-140924 as the general formula (I).
As the amount to be added of the light-to-heat conversion material, when the intermediate layer does not have the unevenness-forming function, the solid weight ratio of the light-to-heat conversion material to the binder is preferably 1:20xcx9c2:1 (light-to-heat conversion material: binder) and more preferably 1:10xcx9c2:1. However, when the intermediate layer does have the unevenness-forming function, the solid weight ratio of the total amount of the light-to-heat conversion material and the particulate material to the binder (i.e., xe2x80x9clight-to-heat conversion material+particulate materialxe2x80x9d: binder) is preferably included in the above ranges. When the amount of the binder is too small, the cohesion of the intermediate layer is decreased and when the formed image is transferred to the image receiving sheet, the intermediate layer is apt to be transferred thereto together with the formed image, which causes color mixing of the image. When the amount of the binder is too large, the thickness of the intermediate layer must be made large in order to attain the desired light absorption coefficient, resulting in a decrease in sensitivity.
When the intermediate layer has the unevenness-forming function, the thickness of the intermediate layer is as explained in the xe2x80x9c2) Material Necessary for light-to-Heat Conversion Functionxe2x80x9d section When the intermediate layer does not have the unevenness-forming function, the thickness of the intermediate layer is preferably 0.03 xcexcmxcx9c0.8 xcexcm and more preferably 0.05 xcexcmxcx9c0.3 xcexcm. It is preferable that the intermediate layer having the light-to-heat conversion function has a maximum absorbance (optical density) of 0.1xcx9c1.3 (more preferably 0.2xcx9c1.1) in the wavelength range of 700xcx9c2000 nm.
3) Method of Forming Intermediate Layer
The intermediate layer is provided, as described above, by preparing the coating solution in which the particulate material and/or the light-to-heat conversion material and the binder are dissolved, coating the coating solution on the substrate or the cushion layer which will be described later and drying the coating solution. Examples of the solvent for preparing the coating solution include 1,4-dioxane, 1,3-dioxolane, dimethyl acetate, N-methyl-2-pyrrolidone, dimethyl sulfoxide, demethylformamide, dimethylacetoamide, xcex3-butyrolactone and the like. Coating and drying can be carried out by a general coating and drying method.
In the heat transfer sheet of the present invention, it is preferable that the cushion layer is interposed between the substrate and the ink layer or between the substrate and the intermediate layer when the heat transfer sheet has an intermediate layer. By providing the cushion layer on the substrate, bad effects such as waviness of the substrate and the like can be eliminated. At the same time, when the heat transfer sheet and the image receiving sheet are superposed with each other and the image is transferred, the adhesion between the heat transfer sheet and the image receiving sheet can be further improved.
The cushion layer, which can be provided in the heat transfer sheet of the present invention, means a layer with cushioning ability. It is preferable that the cushion layer has 1xcx9c250 kg/mm2 of elastic modulus at 25xc2x0 C. or 15xcx9c500 of penetration determined in JIS K2530-1976.
Preferable materials for forming the cushion layer include, for example, ethylene-vinyl acetate copolymer, ethylene-ethyl acrylate copolymer, polybutadiene resin, styrene-butadiene copolymer (SBR), styrentylene-butene-styrene copolymer (SEBS), acrylonitrile-butadiene copolymer (NBR), polyisoprene resin (IR), styrene-isoprene copolymer (SIS), acrylic ester copolymer, polyester resin, polyurethane resin, acrylic resin, butyl rubber, polynorbomene and the like.
Even if the resin alone is not sufficiently cushiony, by adding various additives into the resin, it is possible to ensure the desired cushiony property of the resin and use it as a material for the cushion layer. Examples of the additives include low melting material such as wax and the like, plasticizer and the like. Specifically, the additives include phthalic ester, adipic ester, glycol ester, fatty add ester, phosphoric ester, chlorinated paraffin and the like. Moreover, various additives disdosed in Practical Handbook of Additives for Plastics and Rubber, (Puasuchikku Oyobi Gomu Yo Tenkazai Jitsuyo Ichiran) (Kagaku Kogyo-Sha (1970)) may be used. The amount to be added of these additives may be appropriately adjusted in accordance with the combination with the resin and the additives and therefore is not especially limited. Generally, the amount to be added of the additives is preferably less than 10% by weight and more preferably less than 5% by weight.
The thickness of the cushion layer is preferably 1xcx9c30 xcexcm, more preferably 1xcx9c20 xcexcm and most preferably 1xcx9c10 xcexcm. When the thickness of the cushion layer is less than 1 xcexcm, the effect provided by the cushion layer cannot be attained, which is not preferable.
The cushion layer can be provided by applying the coating solution, in which the above materials are dissolved or dispersed like latex in an appropriate solvent to the substrate and drying the coating solution Coating and drying can be carried out by a conventional coating and drying method.
A heat sensitive peeling layer may be provided under the ink layer of the heat transfer sheet of the present invention. The heat sensitive peeling layer includes a heat sensitive material which, by the action of heat from a thermal head or heat generated from the light-to-heat conversion material of the intermediate layer, generates gas or adhesion water or the like so as to weaken the adhesion strength between the ink layer and the substrate or the cushion layer or the intermediate layer. Examples of such heat sensitive materials include a compound which decomposes or degenerates by heat to generate gas (i.e., a polymer or low molecular compound), a compound which absorbs or adsorbs a large amount of easily vaporizable gases such as water or the like, and the like. These compounds may be used in combination.
Examples of polymers which decompose or degenerate by heat to generate gas include auto-oxidizable polymers such as nitrocellulose, halogen-containing polymers such as chlorinated polyolefin, chlorinated rubber, polychlorinated rubber, polyvinyl chloride, polyvinylidene chloride. Examples of compounds which generate adhesion water or the like include acrylic polymer such as polyisobutyl methacrylate into which a volatile compound such as water is adsorbed, cellulose esters such as ethyl cellulose into which a volatile compound such as water is adsorbed, natural polymer compounds such as gelatin into which a volatile compound such as water is adsorbed, and the like. Examples of low molecular polymer compounds which decompose or degenerate by heat to generate gas include compounds which exothermically decompose to generate gas such as diazo compounds and azide compounds. The decomposition and degeneration and the like of the heat sensitive material by heat is preferably carried out below 280xc2x0 C. and more preferably below 230xc2x0 C.
When a low molecular compound is used as the heat sensitive material of the heat sensitive peeling layer, the low molecular compound is desirably combined with a binder. As the binder, not only polymers which themselves decompose or degenerate by heat to generate gas, but also normal polymers which do not have such properties may be used. When a heat sensitive low molecular compound and a binder are used in combination, the weight ratio of the heat sensitive low molecular compound to the binder is preferably 0.02:1xcx9c3:1 and more preferably 0.05:1xcx9c2:1. It is desirable that the heat sensitive peeling layer coats almost the entire surface of the substrate or the intermediate layer and the thickness thereof is usually 0.03 xcexcmxcx9c1 xcexcm and preferably 0.05 xcexcmxcx9c0.5xcexcm.
The heat sensitive peeling layer decomposes and degenerates by the action of heat generated from the thermal head or heat generated from the light-to-heat conversion material of the intermediate layer to generate gas. Then, due to this decomposition or generation of gas, the heat sensitive peeling layer is partially eliminated or cohesive failure is generated within the heat sensitive peeling layer so that the bonding strength between the substrate, the cushion layer, or the intermediate layer and the ink layer is decreased. As a result, depending on the action of the heat sensitive peeling layer, a portion thereof adheres to the ink layer and appears on the surface of the finally formed image, and becomes the cause of color mixing in the image. Accordingly, in order to prevent visible color mixing of the formed image being caused, even if his type of transfer of the heat sensitive peeling layer is generated, it is desirable that the heat sensitive peeling layer is almost totally colorless, i.e., exhibits high transparency with respect to visible light Specifically, the light absorption coefficient is preferably less than 50%, and more preferably less than 10% with respect to visible light.
The heat transfer sheet of the present invention may be structured such that instead of providing an independent heat sensitive peeling layer, the intermediate layer coating solution into which the heat sensitive material is added is applied to the substrate or the cushion layer to form an intermediate layer which has both the function of the heat sensitive peeling layer and the function of the intermediate layer.
In the heat transfer sheet of the present invention, the surface of the ink layer is made uneven to a certain degree so as to ensure the vacuum adhesion uniformity with the image receiving sheet, and the particulate material having the function of making the surface of the ink layer uneven is not contained in the ink layer. Therefore, the particulate material does not exist on the surface of the heat transfer sheet, and thus, images with high quality can be formed without problems such as image voids or deterioration of the hue of the image.
In the heat transfer sheet of the present invention, the entire heated ink layer is preferably transferred to the image receiving sheet during heat-transfer. If the intermediate layer having absorption in the visible range is transferred to the sheet, problems such as color impurity and the like may be caused, which is not preferable.
In the heat transfer sheet in which the intermediate layer having the light-to-heat conversion function is formed, it is preferable that the light-to-heat-reflection-prevention layer is provided on the opposite surface of the substrate to that on which the ink layer is provided. By providing the light-reflection-prevention layer, when the laser light irradiation operation, in which the laser light is irradiated imagewise onto the surface of the heat transfer sheet, is carried out, image turbulence or a decrease in the resolution due to irregular reflection of light can be avoided.
The light-reflection-prevention layer is generally formed by superposing materials having different indexes of reflection so as to obtain the light-reflection-prevention effect and this method is efficient in the present case. Examples of materials having the above effect include sulfides such as SnS, InS, GeS and the like, oxides of In, Sn, Te, Ga, Si, and the like.
In order to prevent scratches being formed, on a surface of the ink layer of the heat transfer sheet of the present invention, as occasion demands, a protective cover film (for example, polyethylene terephthalate sheet, polyethylene sheet, or the like) may be superposed thereon or an image receiving sheet may be superposed thereon in advance.
Generally, the image receiving sheet is formed such that on an ordinary sheet-like substrate such as a plastic sheet, a metallic sheet, a glass sheet, a paper or the like, one or two or more image receiving layers are disposed. Examples of the plastic sheet include a polyethylene terephthalate sheet, a polycarbonate sheet, a polyethylene sheet, a polyvinyl chloride sheet, a polyvinylidene chloride sheet, a polystyrene sheet, and a styrene/acrylonitrile copolymer sheet and the like. Examples of the paper include printing paper, coated paper and the like. The thickness of the substrate of the image receiving sheet is usually 10 xcexcmxcx9c400 xcexcm, and preferably 25 xcexcmxcx9c200 xcexcm. In order to improve adhesion between the substrate and the image receiving layer or between the substrate and the ink layer of the heat transfer sheet, the surface of the substrate may be treated by a glow discharge treatment, a corona discharge treatment or the like.
As mentioned previously, it is preferable to provide one, or two or more image receiving layers on the surface of the substrate in order to facilitate the transfer and fixation of the ink layer of the heat transfer sheet of the present invention to the surface of the image receiving sheet. The image receiving layer is mainly formed from an organic polymer binder. The binder is preferably a thermoplastic resin. Examples of the thermoplastic resin include homopolymers or copolymers of acrylic monomers such as acrylic acid, methacrylic acid, acrylic ester, methacrylic esters and the like, cellulosic polymers such as methyl cellulose, ethyl cellulose, and cellulose acetate, homopolymers and copolymers of vinyl-based monomers such as polystyrene, polyvinylpyrrolidone, polyvinyl butyral, and polyvinyl alcohol and the like, condensed polymers such as polyesters and polyamides, and rubber-based polymers such as butadiene/styrene copolymers. In order to obtain an appropriate adhesive strength between the image receiving layer and the ink layer, the binder of the image receiving layer is preferably a polymer whose glass transition temperature (Tg) is 90xc2x0 C. or less. Moreover, it is preferable to add a plasticizer to the image receiving layer in order to adjust the glass transition temperature of the image receiving layer.
A laminate of the heat transfer sheet of the present invention and the image receiving sheet is easily obtained by superimposing the ink layer side of the heat transfer sheet and the image receiving side (i.e., the image receiving layer side) of the image receiving sheet with each other and passing them though a press-and-heat roller. In this case, the heating temperature is preferably 130xc2x0 C. or less, and more preferably 100xc2x0 C. or less.
Next, an image forming process using the heat transfer sheet of the present invention will be described.
Firstly, the ink layer of the heat transfer sheet and the image receiving sheet are placed in contact with each other or extremely close to each other (hereinafter, these materials in such a state are referred to as xe2x80x9can image-forming-laminatexe2x80x9d). In the case of a heat transfer sheet in which the intermediate layer having the light-to-heat conversion function is not formed, heat is applied imagewise to the surface of the image-forming-laminate by a thermal head (printing operation), then the image receiving sheet is peeled from the heat transfer sheet to form the image receiving sheet to which the heated region of the ink layer has been transferred. On the other hand, in the case of a heat transfer sheet in which the intermediate layer having the light-to-heat conversion function is formed, laser light is irradiated imagewise onto the surface of the image-forming-laminate in a time series (laser light illumination operation), and thereafter the image receiving sheet is peeled from the heat transfer sheet to obtain the image receiving sheet to which the region subjected to laser light illumination has been transferred.
The printing operation and the laser light illumination operation are usually carried out as follows. Firstly, the image receiving sheet side of the image-forming-laminate is adhered to the surface of a recording drum (which is a rotatable drum with the vacuum-forming mechanism being provided within the interior thereof and a plurality of fine openings being formed at the surface thereof) by vacuum pressure and heat is applied thereto from the outer side, i.e., the heat transfer sheet side, using a thermal head, or alternatively, laser light is irradiated thereon The thermal head or the irradiation of laser light are scanned reciprocally in the transverse direction of the recording drum and the recording drum is rotated at a constant angular velocity during the printing operation or the laser light illumination operation. If the above-described recording drum is not used, the recording may be carried out by scanning on a flat surface using the thermal head or the output head of the laser light.
Examples of the laser light include direct laser lights including gas lasers such as argon ion laser light, helium/neon laser light, helium/cadmium laser light and the like, solid-state laser lights such as YAG laser light and the like, semiconductor laser light, dye laser light, excimer laser light, and the like. Alternatively, light, generated by halving the wavelength of the above-mentioned laser lights through a secondary harmonics element, can also be used. In the image forming method using the heat transfer sheet of the present invention, considering the output power, ease of modulation, and the like, it is preferable to use the semiconductor laser. Further, in the image forming process using the heat transfer sheet of the present invention, the laser light is preferably irradiated such that the beam diameter on the light-to-heat conversion layer is 5xcx9c50 xcexcm (particularly preferably 6xcx9c30 xcexcm). The scanning speed is preferably 1 m/second or greater, and particularly preferably 3 m/second or greater.
The image forming method using the heat transfer sheet of the present invention can be used advantageously not only for forming a black mask or a monochromatic image but also for forming a multicolor image.
Namely, in a first aspect of the method of forming a multicolor image, the following steps are carried out Firstly, the image receiving material is fixed on a rotating drum of a recording apparatus by the vacuum reduced pressure method, and the heat transfer material is Laminated on the image receiving material by the vacuum reduced pressure method such that an image receiving layer of the image receiving material and an image recording layer (hue 1) of the heat transfer material contact each other. Next, laser light modulated on the basis of digital signals of a color-separated image of the original image is irradiated from the substrate side of the heat transfer material while the drum is rotated. Thereafter, the heat transfer material is peeled from the image receiving material with the image receiving material being fixed on the drum. On the image receiving material on which the image of hue 1 has been recorded, heat transfer materials of hues 2,3, and, if necessary, 4 are laminated, subjected to laser recording and peeled by the same method as the above-described method, consecutively. In this way, an image receiving material on which a multicolor image is formed can be obtained.
In order to obtain a color proof image on printing paper, the following steps are carried out. The image receiving material on which a multicolor image has been formed by the above-described steps is laminated such that the image surface thereof contacts a printing paper. Then, the image receiving material and the printing paper are passed through a laminator or the like, heated and pressed, and the printing paper is peeled from the heat transfer image receiving material so that the image is transferred to the printing paper together with the image receiving layer.
In a second aspect of the method of forming the multicolor image, the following steps are carried out In a second aspect of the method of forming the multicolor image, a laminate is prepared for each color by laminating a heat transfer material having an ink layer which contains a coloring agent representing the hue of that color with an image receiving material. In a case of a three-color image, for example, three laminates are required, and in a case of four-color image, four laminates are required. Laser light is irradiated on each of the laminates on the basis of digital signals of the color image corresponding to the laminate obtained via a color separation filter. Thereafter, the heat transfer material is peeled from the image receiving material in each laminate. Color-separated images of respective colors are independently formed on the respective heat transfer image receiving materials, and thereafter, each of color-separated images is successively transferred on the actual substrate, such as a separately prepared printing paper or the like or a similar substrate, so that the image is formed.